Boundary conditions and Berry phase in magnetic nanostructures

The effect of micromagnetic boundary conditions on the Berry curvature and topological Hall effect in granular nanostructures is investi- gated by model calculations. Both free surfaces and grain boundaries between interacting particles or grains affect the spin structure. The Dzyaloshinskii-Moriya interactions yield corrections to the Erdmann-Weierstrass boundary conditions, but the Berry curvature remains an exclusive functional of the local spin structure, which greatly simplifies the treatment of nanostructures. An explicit example is a model nanostructure with cylindrical symmetry whose spin structure is described by Bessel function and which yields a mean-field-type Hall-effect contribution that can be related to magnetic-force-microscopy images

Magnetism of Ta Dichalcogenide Monolayers Tuned by Strain and Hydrogenation

The effects of strain and hydrogenation on the electronic and magnetic properties of monolayers of Ta based dichalcogenides (TaX2; X = S, Se, Te) are investigated using density-functional theo-ry. We predict a complex scenario of strain-dependent magnetic phase transitions involving par-amagnetic, ferromagnetic, and modulated antiferromagnetic states. Covering one of the two chalcogenide surfaces with hydrogen switches the antiferromagnetic/nonmagnetic TaX2 mono-layers to a semiconductor. Our research opens new pathways towards the manipulation of mag-netic properties for future optoelectronics and spintronics applications.Comment: 13 pages, 5 figure

Structural and magnetic properties of Pr-alloyed MnBi nanostructures

The structural and magnetic properties of Pr-alloyed MnBi (short MnBi-Pr) nanostructures with a range of Pr concentrations have been investigated. The nanostructures include thin films having Pr concentrations 0, 2, 3, 5 and 9 atomic percent and melt-spun ribbons having Pr concentrations 0, 2, 4 and 6 percent respectively. Addition of Pr into the MnBi lattice has produced a significant change in the magnetic properties of these nanostructures including an increase in coercivity and structural phase transition temperature, and a decrease in saturation magnetization and anisotropy energy. The highest value of coercivity measured in the films is 23 kOe and in the ribbons is 5.6 kOe. The observed magnetic properties are explained as the consequences of competing ferromagnetic and antiferromagnetic interactions

Magnetic Anisotropy of a Single Cobalt Nanoparticle

Using a new microSQUID set-up, we investigate magnetic anisotropy in a single 1000-atoms cobalt cluster. This system opens new fields in the characterization and the understanding of the origin of magnetic anisotropy in such nanoparticles. For this purpose, we report three-dimensional switching field measurements performed on a 3 nm cobalt cluster embedded in a niobium matrix. We are able to separate the different magnetic anisotropy contributions and evidence the dominating role of the cluster surface.Comment: 4 pages, 8 figure

Exchange through nonmagnetic insulating matrix

Exchange interactions between hard-magnetic particles in a nonmagnetic matrix are investigated by model calculations. A Landau–Ginzburg approach is developed to describe the net exchange interactions between spheres of arbitrary diameters. Introducing cylindrical coordinates and integrating over the surfaces of the adjacent spheres yields an exchange coupling which decreases with a decay length depending on interatomic exchange, intra-atomic exchange, and temperature. Typically, the decay length does not exceed a few interatomic distances. The decay is exponential but also contains a prefactor depending on the surface curvature of the grains. It increases with decreasing curvature, but this dependence is only a small correction to the leading exponential term

Magnetic properties of La 0.6 Sr 0.4 MnO 3 thin films on SrTiO 3 and buffered Si substrates with varying thickness

La0.60Sr0.40MnO3 (LSMO) thin films of varying thickness from 12 to 55 nm were deposited using the pulsed-laser deposition technique onto single-crystalline SrTiO3 (STO) and STO-buffered Si substrates. The Tc of LSMO filmsgrown on STO-buffered Si substrates decreases faster than films directly grown on STO with decreasing film thickness. The LSMO/STO film with thickness of 55 nm shows Tc at about 360 K, which is close to the bulk value, whereas TcLSMO film on STO-buffered Si film of similar thickness is reduced to 320 K. This difference is attributed to the strain and interfacial disorders in LSMO film on STO/Si. The filmsurface morphology is influenced by the film thickness. Oxygenation of LSMO films on STO-buffered Si affects the Tc minimally but improved the overall magnetization of the films due to better oxygenation, which is also the case for postannealing the sample at elevated temperatures. The thermomagnetic history effects observed in LSMO films of STO-buffered Si indicate the presence of inhomogeneity, mostly at the interface, which influences the magnetic properties significantly

Structure and magnetic properties of nanostructured Dy/transition-metal multilayered films

We report the results of magnetic and microstructural studies for T/Dy (T=Fe, Co, Ni) compositionally modulated films prepared in a multiple-gun sputtering system. The perpendicular anisotropy and magnetization were measured systematically for X-Å Fe/Y-Å Dy and X-Å Co/Y-Å Dy films. The layer-thickness dependence of the magnetization for Co/Dy and Fe/Dy was interpreted in terms of the antiparallel coupling between transition-metal and Dy magnetic moments. For Co/Dy films the ranges of X and Y required for perpendicular anisotropy were determined. A comparision of the structural and magnetic properties of Ni/Dy, Co/Dy, and Fe/Dy is given and the origin of the perpendicular anisotropy is discussed. Journal of Applied Physics is copyrighted by The American Institute of Physics

Quasi-long-range order in the random anisotropy Heisenberg model: functional renormalization group in 4-\epsilon dimensions

The large distance behaviors of the random field and random anisotropy O(N) models are studied with the functional renormalization group in 4-\epsilon dimensions. The random anisotropy Heisenberg (N=3) model is found to have a phase with the infinite correlation radius at low temperatures and weak disorder. The correlation function of the magnetization obeys a power law < m(x) m(y) >\sim |x-y|^{-0.62\epsilon}. The magnetic susceptibility diverges at low fields as \chi \sim H^{-1+0.15\epsilon}. In the random field O(N) model the correlation radius is found to be finite at the arbitrarily weak disorder for any N>3. The random field case is studied with a new simple method, based on a rigorous inequality. This approach allows one to avoid the integration of the functional renormalization group equations.Comment: 12 pages, RevTeX; a minor change in the list of reference

Magnetic and structural properties of CoCrTa films and multilayers with Cr

We report our studies of epitaxial growth of CoCrTa films on Cr underlayers and the properties of CoCrTa/Cr multilayers. The coercivity, Hc, strongly depends on Ta composition, sputtering conditions, and the thicknesses of the magnetic layer and Cr underlayer. An Hcvalue of 1300 Oe was obtained for a Ta composition of 2 at. %, a Cr underlayer thickness of 4000 Å, and a magnetic layer thickness of 400 Å. The x-ray data show that the high Hc occurs when crystallites of the Cr underlayer and CoCrTa layer are aligned with the Cr (200) and CoCrTa (110) planes in the film plane. Thus, the c axis of the CoCrTa lies essentially in the plane of the film. When the thickness of the magnetic layer increases above 1000 Å the c axis begins to tip out of the film plane. The basal plane lattice parameter varies roughly linearly with Ta content up to 13 at. %. For the CoCrTa/Cr multilayered films, Hc values up to 1200 Oe were obtained although the c-axis orientation of the magnetic layer becomes somewhat dispersed. Models for the dependence of magnetization reversal on microstructure are discussed. Journal of Applied Physics is copyrighted by The American Institute of Physics. Magnetic and structural properties of CoCrTa films and multilayers with Cr

Structural and magnetic characterization of batch-fabricated nickel encapsulated multi-walled carbon nanotubes

We report on the growth and fabrication of Ni-filled multi-walled carbon nanotubes (Ni-MWNTs) with an average diameter of 115 nm and variable length of 400 nm-1μm. The Ni-MWNTs were grown using template-assisted electrodeposition and low pressure chemical vapor deposition (LPCVD) techniques. Anodized alumina oxide (AAO) templates were fabricated on Si using a current controlled process. This was followed by the electrodeposition of Ni nanowires (NWs) using galvanostatic pulsed current (PC) electrodeposition. Ni NWs served as the catalyst to grow Ni-MWNTs in an atmosphere of H2/C2H2 at a temperature of 700º C. Time dependent depositions were carried out to understand the diffusion and growth mechanism of Ni-MWNTs. Characterization was carried out using scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), Raman spectroscopy and energy dispersive x-ray spectroscopy (EDX). TEM analysis revealed that the Ni nanowires possess a fcc structure. To understand the effects of the electrodeposition parameters, and also the effects of the high temperatures encountered during MWNT growth on the magnetic properties of the Ni-MWNTs, vibrating sample magnetometer (VSM) measurements were performed. The template-based fabrication method is repeatable, efficient, enables batch fabrication and provides good control on the dimensions of the Ni-MWNT